Offset printing presses are provided with a dampening system in which a fluid transfer roller transfers water to a plate cylinder.
The amount of water carried by the fluid transfer roller and delivered to the plate cylinder can be critical to the proper operation of the offset printing press. When a fluid transfer roller does not pick up a sufficiently even flat film of water, printing can occur on areas not intended to be printed and there can be a buildup of ink on the rollers which requires that the offset printing press be shut down to permit cleaning. Both printing errors and shutdowns, of course, are costly and time-consuming. Conversely, when too much water is delivered by the fluid transfer roller to the plate cylinder, the ink can be overly diluted on the plate cylinder and the ink may become emulsified. In addition, it also can result in printing on areas which are not intended to be printed. Once again the result is that the efficiency and the performance of the printing system suffers.
One type of fluid transfer roller used in the past in dampening systems was made of steel and had a surface which was either chromium plated or flame sprayed with a metal oxide, such as aluminum oxide, and which was considered hydrophilic. This type of fluid transfer roller did not have a reservoir and because it was relatively heavy it required a large motor for operation. In addition, such steel rollers tended to corrode which could cause an ink buildup on the water rollers.
The most common currently used fluid transfer rollers are elastomeric rollers which are covered with a paper cover or cotton sleeve which creates a fluid reservoir which allows for the proper continual wetting of the non-image areas of the printing plate. The covers and sleeves make the roller surface more hydrophilic. The liquid storage capability of these covers and sleeves provides rollers with a reservoir which allows for the acceptance of excess water when not needed by the printing plate and which supplies more water to the printing plate when the demand is increased. Due to the intermittent needs of the printing plate it is a requirement that these rollers not only act as a reservoir but that they also transfer water to the printing plate when required.
One problem with using a paper cover or cotton sleeve is that to install the cover or sleeve the press has to be shut down, the elastomeric rollers removed from the press, the covers or sleeves positioned over the elastomeric rollers and then the rollers reinstalled back into the press. Because of the considerable down time which occurs, this can be a very costly procedure. In addition, although the sleeves and covers are effective, they are not very durable and they have to be replaced often due to damage, ink contamination and/or wear.
A need exists for a fluid transfer roller which does not possess the disadvantages of the prior art fluid transfer rollers.